Do Pilots Understand How An Airfoil Generates LIft? (Survey Says...)

[PeterNSteinmetz]

Was thinking about this a bit more and of course this will always depend on the audience.

With a 14 year old taking lessons to fly gliders, I think even the pressures creating forces to accelerate parcels of air may be a bit much. That is where the simple demos may be better or just saying there is a higher and lower pressure and you will learn more about it later may work better.

 

[Everskyward]

I'm missing something here...

A sail generates lift just like the wing on airplane. Sailing downwind, sails tend to be at least partially stalled and that is why downwind is slow. Trimming a spinnaker you want to keep it eased as much as possible to maintain as much attached flow as possible to maximize "lift".
The keel generates lift just like the wing on an airplane.
The fundamental physics are the same. One just trims the sails to get the maximum forward component of force.

The way I see it, a sail only generates lift like a wing when sailing close hauled, on a close reach (somewhat upwind). When sailing downwind, the airflow is pushing the sail. The relative wind is from below. On an airplane, the wing would be stalled and no lift would be generated. In any case, all the forces involved, including water on the keel, seem much more complex than on an airplane. Yet people blithely explain it as "just like an airplane wing".

 

[Palmpilot]

A pure round 'chute with no vents is not going to generate lift. And, as a result it has to be bigger. And, you ain't gonna find one on the market.

In that case I guess the force that slows the descent would be called drag?

 

[AKiss20]

Does one normally explain Newton’s laws by saying that the acceleration of an object causes the the force on it? Or the other way around?

You are falling into a common trap many do when thinking about subsonic aerodynamics. It is not a good mental model to think of causality in this way. Your statement basically boils down to "Does the pressure gradient cause the flow turning/streamline curvature or does the flow turning/streamline curvature cause the pressure gradient?" The answer is there is not arrow of causality here, but rather a compatibility of the resultant flow pattern/solution with the governing equations and the boundary conditions imposed by the wing. While with time and experience one can develop deep physical intuition for these types of systems and flows, thinking of it causally like this often leads one into circles because as I said, there isn't a direct arrow of causality but rather compatibility. As to *why* the flow/streamlines must curve, see my next post.

 

[AKiss20]

Where the definition of circulation is, of course, the line integral of the component of the flow velocity along a closed curve. The theorem tells us that the magnitude of lift is proportional to the magnitude of the circulation and that the magnitude of the circulation is determined by the Kutta condition - the velocity of the flow at a sharp trailing edge must be finite. I suspect that this is less than intuitive for a non-technical (math, physics, engineering) person.

This is a correct statement but you have sort of gone one turtle down and introduced circulation but have not discussed where this concept of circulation comes from (@Capt. Geoffrey Thorpe, note that while I am confident you may know what I am about to discuss, I feel it adds to the discussion to write it out, please do not feel that I am lecturing to you). The answer is ultimately viscosity and the no-slip boundary condition on the wing surface imposed by viscosity. One can model the flow inviscidly, and doing so is a common practice as it can very accurately capture the lift on an airfoil, but by doing so you must impose a source of circulation "buried within the airfoil" by fiat to satisfy the slip boundary condition at the airfoil surface (no normal velocity, aka the flow cannot penetrate the airfoil surface). There is an infinite number of solutions for the inviscid problem, but the one that actually represents reality is the one that satisfies the extra condition of a stagnation point at the trailing edge, aka the Kutta condition you mentioned as shown in the figure below

But where does this condition come from? Why do we impose it seemingly arbitrarily? The answer is viscosity. Flow in the real world cannot negotiate the very small radius turning required by all the other solutions to the inviscid problem. With viscosity, the flow will separate, imposing the condition that the flow must stagnate at the trailing edge. So we take insight from viscous flow and apply it to select the one physical solution of the infinite solutions to the inviscid problem. Furthermore, as I said before, in the inviscid problem we must impose a circulation "buried within the airfoil" to satisfy the slip boundary condition at the airfoil surface (and again the magnitude of the circulation comes from the Kutta condition) but this is a mathematical imposition to satisfy the boundary conditions. There is no physical mechanism to produce this circulation in the inviscid world, it is simply one we imposed. Where does the circulation come from in the real world? The answer is again viscosity. The boundary layers on the suction and pressure side of the airfoil in the viscous case are continuously generating vorticity. When this vorticity field is spatially integrated via Stokes' theorem, you get the net circulation of the airfoil. So we see that viscosity is truly the key driver to why the flow pattern around the airfoil is the way it is. While we can model the flow around the airfoil inviscidly and derive useful information from doing so, we must use conditions derived from the fact that the true underlying flow we are modeling is viscous to impose a real world matching condition and select the physically realizable solution.

 

[Piperonca]

What a wonderful thread! Better than my aero courses in college.

 

[Capt. Geoffrey Thorpe]

This is a correct statement but you have sort of gone one turtle down and introduced circulation but have not discussed where this concept of circulation comes from

Quite correct. My issue with circulation is that it pretty much sucks as an intuitive "feel good" topic. Thus, I believe that circulation and Bernoulli are generally best avoided if you are trying to provide a simplistic explanation of how your avgas dollars are converted into altitude.
My original objective was to get a better idea of just what fraction of the pilot world believes in the nonsense ideas that were in the survey - but it would have been better to have a "none of the above" tick box. Oh, well.
But yes, we need viscosity to make things work in the real world in spite of the fact that it is so convenient to assume that air is inviscid (for the most part).

Also, to change the subject, I did find the previously mentioned paper by Prandtl that had an illustration that perhaps may have been an inspiration for the "equal transit" nonsense (again, Prandtl does not make any kind of claim supporting that idea) https://ntrs.nasa.gov/citations/19930091180


The figure is intended to illustrate how viscosity can introduce rotation in a fluid flow.

 

[Capt. Geoffrey Thorpe]

What a wonderful thread! Better than my aero courses in college.

You are one sick puppy.

 

[Piperonca]

You are one sick puppy.

My eyeballs are rolling like those Flettner rotors...I can't be alone in this.

 

[Capt. Geoffrey Thorpe]

The way I see it, a sail only generates lift like a wing when sailing close hauled, on a close reach (somewhat upwind). When sailing downwind, the airflow is pushing the sail. The relative wind is from below. On an airplane, the wing would be stalled and no lift would be generated. In any case, all the forces involved, including water on the keel, seem much more complex than on an airplane. Yet people blithely explain it as "just like an airplane wing".

Yes, there are some complications, and downwind, you do operate with flow separation.

 

[AKiss20]

Quite correct. My issue with circulation is that it pretty much sucks as an intuitive "feel good" topic. Thus, I believe that circulation and Bernoulli are generally best avoided if you are trying to provide a simplistic explanation of how your avgas dollars are converted into altitude.

Fair but as you have noted before, there really is no good one-liner that provides any form intuition for this so honestly I wish pilot education would just drop trying to explain it. Much more important to educate pilots on the consequences of airfoil flows than the underlying physical mechanisms that drive them. Examples we already teach would be critical angle of attack resulting in separation rather than airspeed and consequently why maneuvering speed increases with weight rather than decreases. Rather than give them a false view of aerodynamics for no real purpose, simply give them actual resources if they are interested in diving into the math and physics, otherwise give them information that is relevant to piloting an aircraft and leave it at that.

 

[PeterNSteinmetz]

You are falling into a common trap many do when thinking about subsonic aerodynamics. It is not a good mental model to think of causality in this way. Your statement basically boils down to "Does the pressure gradient cause the flow turning/streamline curvature or does the flow turning/streamline curvature cause the pressure gradient?" The answer is there is not arrow of causality here, but rather a compatibility of the resultant flow pattern/solution with the governing equations and the boundary conditions imposed by the wing. While with time and experience one can develop deep physical intuition for these types of systems and flows, thinking of it causally like this often leads one into circles because as I said, there isn't a direct arrow of causality but rather compatibility. As to *why* the flow/streamlines must curve, see my next post.

Agreed. That was sort of my point relative to the video under discussion. It does not make sense to say that the acceleration causes either the force or the pressure difference, as the video does.

 

[Everskyward]

Yes, there are some complications, and downwind, you do operate with flow separation.
View attachment 95849 View attachment 95850

Those are great illustrations!

 

[Palmpilot]

The way I see it, a sail only generates lift like a wing when sailing close hauled, on a close reach (somewhat upwind)....

I would think that a beam reach would qualify as well.

 

[Crashnburn]

In my defense, I’ve run across every one of those statements as explanations for lift. I think anything will fly if you can hang a big enough motor on it. But it takes money for the motor, and for the Av-Gas so ultimately money generates lift!

 

[Cap'n Jack]

I would think that a beam reach would qualify as well.

Broad reach, too (can we still use that term? ). Look at how those America's Cup hydrofoils waork back down the course.

 

[Everskyward]

Broad reach, too (can we still use that term? ). Look at how those America's Cup hydrofoils waork back down the course.

I agree that beam reach would still qualify, not sure about broad reach. When does the airflow separate? Depends on the sail, but running definitely wouldn't qualify as working like an airfoil.

 

[Cap'n Jack]

I agree that beam reach would still qualify, not sure about broad reach. When does the airflow separate? Depends on the sail, but running definitely wouldn't qualify as working like an airfoil.

If the sails are cut & trimmed properly, it shouldn't separate. If your sails have streamers, they should still show attached airstream to the leech. That's also why I suggested looking at the America's cup foils. They do a broad reach, very fast. I'm not sure where a broad reach becomes a run....probably when the airflow separates

 

[Everskyward]

If the sails are cut & trimmed properly, it shouldn't separate. If your sails have streamers, they should still show attached airstream to the leech. That's also why I suggested looking at the America's cup foils. They do a broad reach, very fast. I'm not sure where a broad reach becomes a run....probably when the airflow separates

But at some point, it does separate. I'm sure that it depends on the sail and the person's skill at trimming. Note that America's cup foils are not what most people are sailing.

In fact it seems that, unless someone is a serious racer, they're not that worried about trimming. Sure, you want to get the streamers flying aft, but there are numerous ways of adjusting the sails that you can't do to an airplane wing.

 

[IK04]

 

[Cap'n Jack]

But at some point, it does separate. I'm sure that it depends on the sail and the person's skill at trimming. Note that America's cup foils are not what most people are sailing.

In fact it seems that, unless someone is a serious racer, they're not that worried about trimming. Sure, you want to get the streamers flying aft, but there are numerous ways of adjusting the sails that you can't do to an airplane wing.

No, those boats aren't what most people sail, but they do show that a broad reach can still have airflow attachment, such that they can sail faster than the wind, until they transition from a broad reach to a run. An Optimist pram is the other end of the sailing spectrum, where the transition from a reach to a run happens earlier when turning downwind. The hull simply can't go very fast, and the airflow separates (much) earlier then the foil as one turns downwind. A Laser or Sunfish behaves differently on and off a plane on a broad reach; those act like the foil on a plane, and more like the pram when simply displacing water.

 

[Timbeck2]

What happens when you tape a piece of toast, jelly side up on a cat's belly and toss it from above?

Does the cat levitate?

 

[David Megginson]

There was a wonderful article in Scientific American about six months or a year ago. I will try to find it and post the year and month in this thread.

I skimmed it: https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/

A bit of a garbage pop-culture article, trying to create a false dichotomy between Bernoulli's work on fluid dynamics and Newton's laws of motion to add some drama to the article.

Section 3.4 here gives a more-accurate (and shorter) explanation: https://www.av8n.com/how/htm/airfoils.html#sec-upwash-downwash

 

[Hang 4]

Does knowing how the wings create lift make you a better pilot?

 

[David Megginson]

Does knowing how the wings create lift make you a better pilot?

Not really. Understanding pitch stability or yaw-roll coupling helps you fly more proficiently, but I've never seen the point of drilling student pilots on pressure differentials or fluid dynamics.

 

[Capt. Geoffrey Thorpe]

Does knowing how the wings create lift make you a better pilot?

I would think not.

 

[Palmpilot]

For me, learning how things work enhances the joy of living.

 

[ebetancourt]

I really enjoyed this thread, and learned (and remembered) good stuff. We need a sub-thread to discuss how to distill this to create a useful explanation for instruction. I'm going with, "it's a really complicated subject based on fluid dynamics. For our purposes we'll simply call it flow changes driven from the movement of an airfoil through a fluid we call "air," together with a little Newton."

 

[David Megginson]

I really enjoyed this thread, and learned (and remembered) good stuff. We need a sub-thread to discuss how to distill this to create a useful explanation for instruction. "I'm going with, it's a really complicated subject based on fluid dynamics. For our purposes we'll simply call it flow changes driven from the movement of an airfoil through a fluid we call "air, together with a little Newton."

Glad you enjoyed it. It's even simpler than that. Tell students "A wing with a small positive angle of attack creates an area of lower air pressure above it and higher air pressure below it. Past the critical angle of attack, that air starts swirling around more and doesn't do your wing much good any more. That's all you need to know. Now stop daydreaming and watch your altitude!"

 

[WWFeldman]

I knew how a wing generated lift when I was a kid and stuck my hand out the window of the car.

 

[Piperonca]

For me, learning how things work enhances the joy of living.

I couldn’t agree more. Listening to all the aerodynamic theory in classrooms can be dry as a popcorn fart. But put it in context, with some of the the most learned members of this forum jousting vigorously, then all of a sudden it becomes interesting. Who is right? What is the best way to understand? Find it then pass it on, by all means.

 

[Capt. Geoffrey Thorpe]

It does not make sense to say that the acceleration causes either the force or the pressure difference, as the video does.

If you tie a weight to a string and swing it around overhead, does the acceleration of the weight cause the force in the string or does the force in the string cause the acceleration?

 

[bflynn]

I agree that beam reach would still qualify, not sure about broad reach. When does the airflow separate? Depends on the sail, but running definitely wouldn't qualify as working like an airfoil.

I suppose that depends on how your qualify what "working like an airfoil" means.

If I take the purpose of the airfoil as a means to redirect air, then a sail running downwind still qualifies. By blocking the air, it is causing the air to change direction and that change in direction is what pushes the boat forward. That's still an airfoil.

I saw the recent America's Cup races, take a look at the new AC75 "boats" sailing downwind Absolutely an airfoil. They are monstrosities, but they can sail 1.6 times wind speed downwind.

 

[Checkout_my_Six]

so....if I had a treadmill big enough....would it work?

 

[Cap'n Jack]

I saw the recent America's Cup races, take a look at the new AC75 "boats" sailing downwind Absolutely an airfoil. They are monstrosities, but they can sail 1.6 times wind speed downwind.

Were they sailing directly downwind? Or a broad reach that takes then downwind faster than a direct downwind course?
See:
Points of sail
https://en.wikipedia.org/wiki/High-performance_sailing

 

[Capt. Geoffrey Thorpe]

Were they sailing directly downwind?

As you point out dead downwind is slow. One needs to "sail your angles" even on an old wooden boat built in 1946.

 

[bflynn]

More of a broad reach, but not really because the speed changes the relative wind.

BTW, I was wrong about the speed. At one point on the downwind leg, one of the boats was making 43 kts in a 15 kt wind. That isn't right.

 

[Cap'n Jack]

That's how we sail the prairie schooners in Nebraska and the plains

 

[PeterNSteinmetz]

If you tie a weight to a string and swing it around overhead, does the acceleration of the weight cause the force in the string or does the force in the string cause the acceleration?

Seems pretty clear to me that the force on the string accelerates the weight and that without that force the weight continues in a straight line.

You might be alluding to the “centrifugal force” apparently pushing things out in the accelerated rotating reference frame.

But many people complain about that sort of explanation as well and it acts in the opposite direction. I don’t think in the video he was referring to a virtual force felt in the reference frame of the accelerated air.

Overall I thought it was a fairly good video - but would benefit from a new audio track with some corrections.

 

[Bill]

As you point out dead downwind is slow. One needs to "sail your angles" even on an old wooden boat built in 1946.
View attachment 95869

And therein lies the skills of a good downwind sailor. How much angle? Angles add distance, but also add speed, what's the magic angle? Hard to figure out sometimes...